The influence of block copolymer microstructure on the toughness of compatibilized polylactide/polyethylene blends
Introduction
The toughening of polylactide, an environmentally friendly thermoplastic [1], has been investigated using a wide variety of toughening agents, mostly emphasizing biocompatible materials. Successful methodologies include copolymerization strategies [2], [3], [4], plasticization with a miscible component [5], [6], [7], [8] and blending with an immiscible homopolymer [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19] or a block copolymer [13], [14], [20], [21], [22]. However, in the case of immiscible poly(l-lactide) (PLLA) blends, unifying principles that govern toughening are lacking [23].
We previously reported the toughening of PLLA by blending with linear low-density polyethylene (LLDPE) [9], [10], concentrating on the compatibilization with a poly(l-lactide)-polyethylene (PLLA-PE) block copolymer. With a solution blending technique, toughening was only achieved after the PLLA-PE block copolymer was introduced as a compatibilizer [10]. On the contrary, by melt blending the two homopolymers we found that toughened binary blends could be obtained; however, upon addition of the PLLA-PE block copolymer a significant reduction in impact strength variability and an increase in the overall toughness was observed [9]. While the addition of a compatibilizer was useful for preparing toughened PLLA composites, we aim to better understand the role of the compatibilizer. Specifically, we are interested in the influence of crystallinity in the block copolymer both on the interfacial adhesion between the matrix and the dispersed phase, and the distribution of the dispersed phase particles. To uncover the role of these parameters on the composite toughness we will use three polyethylene dispersed phases with differing levels of crystallinity and thus modulus.
Fig. 1 gives a schematic view of the variables that will be explored in this study. Both the PLLA and polyethylenes are commercially available. As compatibilizers, we prepared four polylactide–polyethylene block copolymers that are essentially identical except for their abilities to co-crystallize with the homopolymers (altered through variations in tacticity or defect site density). Of the four block copolymers used, one material contains two semi-crystalline blocks, one is completely amorphous and the other two contain one semi-crystalline component and one amorphous component. In principle, all blocks will be able to mix and entangle with the homopolymers, but the interlocking of the chains into crystallites will only be possible for a subset of the block copolymers. Therefore, the varying abilities of the block copolymers to co-crystallize with the homopolymers can be correlated with the adhesion/mechanical properties [24]. While some improvement in the adhesion due to entanglements is likely [25], a significant improvement is expected when the blocks are able to co-crystallize [26], [27].
In addition to the adhesion between the dispersed and matrix phases, the particle size and distribution of the dispersed phase can significantly affect the resultant toughening. As first proposed by Wu [28], [29], there is a critical distance between neighboring dispersed phase particles known as the critical matrix ligament thickness. Wu suggests that in order to observe toughening one must be able to attain dispersion of the rubber such that the matrix ligament thickness is below the critical value. This critical distance is believed to be related to the overlap of the stress fields which surround the dispersed phase particles. Once you are below the critical matrix ligament thickness and the stress fields are able to interact, the matrix phase will undergo enhanced shear yielding resulting in tough behavior. By preparing polylactide–polyethylene block copolymers with varying crystallinities we hope to modulate the interfacial adhesion while maintaining an adequate dispersion, and thus decouple matrix ligament thickness effects from interfacial adhesion effects.
While the role of interfacial adhesion on toughening has been thoroughly explained, there are system to system variations that have not been reconciled. For example, Wu suggested that only van der Waals adhesion is necessary to achieve toughening since, according to his analysis, the main parameter controlling the toughening is the matrix ligament thickness [28]. In contrast, others have found that increased interfacial adhesion was either detrimental to the impact properties [30], [31] or had no significant effect on the toughening [32], [33], while still others report that good adhesion between the matrix and the rubbery particles are necessary to promote rubber particle cavitation [34]. However, in all of the above instances the dispersed phase properties were not investigated as to potential reasons for the observed effects. While the influence of the dispersed phase properties on the resultant toughening has been explored [35], [36], [37], in these cases the role of interfacial adhesion was essentially ignored. By examining the combined effects of interfacial adhesion and dispersed phase properties on the toughening of PLLA, we aim to uncover generic requirements for the toughening of PLLA with other semi-crystalline polymers. We hope the results of this study can be applied more generally to compatibilized semi-crystalline/semi-crystalline blends [38].
Section snippets
Materials
Commercial grade PLLA (98.7 mol% l-isomer) was supplied by Cargill-Dow Polymer, LLC (Natureworks™). The two commercial grade LLDPEs were obtained from Dow Chemical Company (Engage EG 8100, ethylene/octene copolymer containing 13.2 mol% octene, MFI=1.0 g/10 min, ρ=0.870 g/cc and Engage EG 8540, ethylene/octene copolymer containing 4.8 mol% octene, MFI=1.0 g/10 min, ρ=0.908 g/cc). These homopolymers will be referred to as LLDPE1 and LLDPE2. A commercial grade HDPE was also obtained from Dow Chemical
Blend components
Two linear low-density polyethylenes (LLDPE1 and LLDPE2) and a high-density polyethylene (HDPE) were chosen as the dispersed phase polymers. The two LLDPEs are both ethylene/octene copolymers that differ only in the amount of octene incorporation, which significantly influences the degree of crystallinity (HDPE>LLDPE2>LLDPE1) (Table 1).
The tensile properties of the PLLA and PE homopolymers are also given in Table 1, which includes the ultimate elongation (ɛb), tensile modulus, ultimate tensile
Discussion
Central to this study is the influence of the block copolymer structure on the interfacial adhesion between the dispersed polyethylenes and the matrix PLLA. The adhesion testing results suggest that the ability of the polyolefin in the block copolymer to co-crystallize with the dispersal phase polyethylene results in a significant improvement in the interfacial adhesion (Table 3). The crystallinity of the polylactide block in the block copolymers made little difference in the interfacial
Conclusions
We investigated the influence of interfacial adhesion and dispersed phase properties on the toughening of PLLA. The interfacial adhesion was altered by introducing different block copolymers with varying abilities to crystallize. Direct measurements of interfacial adhesion were obtained using a 180 °C peel test and dual cantilever crack propagation test. Using these tests, we found increased adhesion strength with the PLLA-PE and PLA-PE block copolymers, an effect seemingly due to the
Acknowledgements
The authors would like to thank Laura Crawford for performing the tensile testing of the Engage 8540 and Engage 8100 polyethylenes. The authors also thank Professor Christopher W. Macosko and Dr Yonathan Thio for helpful input during the preparation of this manuscript. The David and Lucile Packard Foundation and the Toyota Motor Company are acknowledged for financial support of this work.
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